The present invention relates to a multilayer optical disc formed of a plurality of information recording sections layered in a multilayer fashion. More specifically, the present invention relates to a multilayer optical disc at least having a phase change recording layer formed of a phase change recording material.
With development of information communication devices, there is a need for optical discs having much more recording capacities as recording medium for this kind of devices. A DVD (Digital Versatile Disc) is an example of optical discs intended for an increased recording capacity. There is provided a read-only DVD with the recording capacity of 8.5 Gbytes by using two layers of information recording section. Further, there is provided a read/write DVD capable of rewriting of information signals with the recording capacity of 5.2 Gbytes and with only one layer of a recording section.
An optical disc with the recording capacity of 8 Gbytes is needed for recording four hours of a television program according to the NTSC (National Television System Committee) system. An optical disc with the recording capacity of 24 Gbytes is needed for recording two hours of a television program according to the high-definition broadcast.
The amount of information to be handled is increasing with the development of digital information communication systems. There is an increasing demand for high-density and high-capacity media for recording a large amount of information Particularly, there is a need for high-density and high-capacity optical discs.
In order to improve the recording density and increase the capacity of information to be recorded on optical discs, it is necessary to shorten the wavelength of laser used for recording or reproducing information signals on an optical disc and to increase a numerical aperture (NA) of an objective lens for condensing the laser on a signal recording surface of the optical disc. In order to record or reproduce information signals from such optical disc designed for high density and capacity, there is proposed a recording and reproducing apparatus which uses laser with the wavelength of approximately 400 nm and an objective lens for condensing this laser with the numerical aperture (NA) of approximately 0.6. Alternatively, in order to comply with high-density and high-capacity optical discs, there is proposed a recording and reproducing apparatus which uses laser with the wavelength of approximately 660 nm and an objective lens for condensing this laser with the numerical aperture (NA) of approximately 0.85. Further, in order to comply with high-density and high-capacity optical discs, there is proposed a recording and reproducing apparatus which uses laser with the wavelength of approximately 400 nm and an objective lens for condensing this laser with the numerical aperture (NA) of approximately 0.85.
There is proposed a DVD configured as a recording and reproducing optical disc designed for high-capacity by forming two layers of an information recording section made of a phase change recording material.
When an optical disc comprises a 2-layer information recording section, each section is composed of a phase change recording material based on stoichiometric compositions such as Ge:Sb:Te=2:2:5 (atomic ratio). In order to further increase the recording capacity and provide a high transfer rate for the optical disc which forms an information recording section by using the phase change recording material, it is necessary to use a crystallization accelerating layer for the information recording section and further use a technique called the absorptivity control.
Here, the crystallization accelerating layer is a dielectric layer which has a capability of accelerating crystallization of the phase change recording material by contact with this material. Accordingly, the crystallization accelerating layer is provided on the optical disc so as to contact the phase change recording material. Materials used for forming the crystallization accelerating layer include dielectric materials such as Si3N4, GeN, etc. which show poor wettability with a molten phase change recording material.
In order to increase the capacity and the density of optical discs, it may be considered to shorten the wavelength of laser used for recording and reproducing information signals, increase the numerical aperture (NA) of an objective lens for condensing the laser, decrease the diameter of a beam spot formed on the optical disc's signal recording surface, and increase the optical disc's rotational speed for a higher transfer rate. These conditions shorten the time for allowing a laser's beam spot to pass a given point on the information recording section. As a result, when the laser is irradiated to the phase change recording material to record an information signal, the time for heating up the recording material becomes short.
To record an information signal on the information recording section using the phase change recording material, it is necessary to crystallize an amorphous region corresponding to the information signal to be recorded. This method records information signals by irradiating the laser to an amorphous portion of the recording material layer using a so-called bias power for heat-melting, and then cooling that portion for crystallization.
When an information signal is recorded on the information recording section using the phase change recording material, an insufficient speed of crystallization for the region melted by irradiation of the laser makes it impossible to record the information signal at a high transfer rate. In order to record information signals on the information recording section using the phase change recording material at a high transfer rate and at a high density, it is necessary to form the recording material layer so as to contact the crystallization accelerating layer for improving the crystallization speed after the melting.
An absorptivity control technique employed for increasing the capacity and the density of optical discs having the information recording section using the phase change recording material controls light absorption coefficient Ac for crystallization and light absorption coefficient Aa for amorphism of a phase change material layer. The technique provides control so as to satisfy Ac/Aa≧1.0 for the relationship between light absorption coefficient Ac for crystallization and light absorption coefficient Aa for amorphism. Desirably, control is provided to satisfy Ac/Aa≧1.2.
Generally, the phase change recording material causes the temperature to rise differently depending on crystalline and amorphous states in response to the supplied energy. The temperature rises rapidly during amorphism.
When additional information is overwritten to the information recording section where an information signal is already recorded, the information signal is recorded by irradiating the laser to a region mixedly comprising an amorphous and crystalline regions under the same condition. Namely, the laser is irradiated simultaneously to the amorphous and crystalline regions to melt and then crystallize these regions for recording the information signal. At this time, when there is the relationship of Ac/Aa<1.0 between light absorption coefficient Ac in a crystalline state and light absorption coefficient Aa in an amorphous state, the amorphous region's ultimate temperature becomes higher than the crystalline region's ultimate temperature. That is, under the condition of Ac/Aa<1.0, a recording mark is formed differently on the phase change recording material layer before recording depending on whether it is crystalline or amorphous. This deteriorates a jitter value as an evaluation function generally used for the reproduction signal evaluation, impairs reproduction characteristics of reproduction signals, and disables reproduction of the recorded information signal with good reproduction characteristics. To solve these problems, there was proposed an absorptivity control technique for providing control so as to satisfy Ac/Aa≧1.0 for the relationship between light absorption coefficient Ac in a crystalline state of the recording material layer and light absorption coefficient Aa in an amorphous state thereof.
The inventors formed a recording layer using the phase change recording material of Ge:Sb:Te=2:2:5 and formed a two-layer information recording section comprising a crystallization accelerating layer provided on the recording layer. Further, the inventors created a multilayer optical disc and examined its characteristic by controlling the relationship between light absorption coefficient Ac in a crystalline state of the above-mentioned recording material layer and light absorption coefficient Aa in an amorphous state thereof.
We irradiated the laser with the wavelength of approximately 400 nm to the optical disc by using an objective lens with the numerical aperture (NA) of 0.85 to obtain recording characteristics for each information recording section. As a result, we found that it is difficult to produce a transfer rate of 30 Mbps or more in one of the two layered information recording sections located at the laser's incident side. This seems to be caused by difficulty of controlling the relationship between light absorption coefficient Ac in a crystalline state of the recording material layer and light absorption coefficient Aa in an amorphous state thereof.
The material of Ge:Sb:Te=2:2:5 is used for the above-mentioned optical disc's recording layer and shows complex indexes of refraction to be approximately (2.0-3.0i) in a crystalline state and approximately (3.0-2.0i) in an amorphous state in response to the laser with the wavelength of approximately 400 nm. The optical disc comprises two layered information recording sections. One section is positioned at a laser-incident side. The other, i.e., the second information recording section, is positioned at a substrate side relative to the laser-incident side. In order to record and reproduce information signals on the second information recording section, it is necessary to set the optical transmittance of 40% or more for the first information recording section located at the laser-incident side. Accordingly, if a metallic layer is available to an optical disc having only one information recording section or to the second information recording section provided at the substrate side of the 2-layer structure, that metallic layer cannot be used as a recording layer for the first information recording section at the laser-incident side.
As mentioned above, the optical disc formed of two recording layers comprising the phase change recording material makes it difficult to provide Ac/Aa≧1.2 as the relationship between light absorption coefficient Ac in a crystalline state of the recording material layer and light absorption coefficient Aa in an amorphous state thereof according to conditions of an optical constant required of the recording material and the optical transmittance required of the laser-incident side. Accordingly, it is very difficult to provide a high transfer rate for each information recording section on the optical disc having a plurality of recording layers using the above-mentioned phase change recording material of Ge:Sb:Te=2:2:5.